The present invention relates in general to a method and a system for judging a milling end point in a charged particle beam milling system for focusing charged particles such as an ion beam or an electron beam to form or mill a fine pattern on a predetermined substrate, and more particularly to a method and a system which are suitable for detecting a milling end point with high accuracy while monitoring secondary ions which are generated from a milled surface by applying charged particles in forming or milling a pattern in the process of manufacturing a semiconductor device such as an LSI or an electronic device such as a liquid crystal display panel which requires a fine pattern.
As for a charged particle beam milling system for focusing a charged particle beam such as an ion beam or an electron beam to irradiate a predetermined substrate with the charged particle beam, thereby forming or milling a fine pattern, the various kinds of methods and systems have been proposed. Now, as the typical example, the description will hereinbelow be given with respect to the case where an ion beam is focused in order to mill an LSI.
When applying the accelerated ion beam to an LSI, the constituent material of the LSI is removed from a surface of the LSI by sputtering. The technology of milling an LSI with an ion beam by controlling the sputtering has been conventionally well known.
In recent years, out of that technology, there is becoming general carrying out the logic modification of a logical LSI or the failure analysis of an LSI in such a way that by employing a focused ion beam using a high luminance ion source such as a liquid metal ion source of Ga or the like for example, the wiring of the LSI is cut, a window is formed through an insulating film overlying the wiring to expose the wiring, and the window is filled with metal by utilizing the CVD (Chemical Vapor Deposition) method to connect the wiring of interest to the different wiring. In this connection, the accuracy of the milling position and the milling depth becomes a problem in order to ensure the milling yield which is required in the respective applications.
The milling position is set with necessary accuracy by making free use of the technology wherein a stage on which an LSI is mounted is subjected to the length measurement to measure the accurate position and the mechanically positional deviation is corrected on the basis of the deflection of the ion beam, the technology wherein the charge-up of the surface of an LSI due to the positive electric charges of the ion beam is neutralized with the negative electric charges of the electrons by applying the electron shower thereto, or the like.
On the other hand, with respect to the milling depth, as described in JP-A-63-164219 for example, there is well known the technology wherein the sputtering rate of the constituent material of an LSI is previously measured and the accurate ion dose is measured during the actual milling process, thereby improving the depth accuracy.
In addition, when applying the ions to the material to be milled, the secondary particles are emitted from the surface to which the ions are applied. The secondary particles contain light, the secondary electrons and the secondary ions and also have the different yields depending on the difference between the construction materials of the layers constituting an LSI. Therefore, there is also well known the technology wherein the secondary particles are detected in order to monitor the progress of the milling, and at a time point when recognizing the predetermined signal change, the milling is stopped, thereby ensuring the milling depth accuracy. Those technologies are described in JP-B-3-28017, JP-A-58-202038, JP-B2-5-14416, JP-A-6-96712 and an article of Journal of Vacuum Science and Technology B6(6), November/December 1988, pp. 2100 to 2103 for example.
During the formation of a pattern, the accurate measurement of the ion dose as the technology for stopping the milling at a predetermined depth is, when milling a single construction material, the powerful control means in which that technology is all we can adopt. In the case where the object of milling is an LSI for example, however, the surface has irregularity which reflects the presence of the underlying wiring. Since if that irregularity is included in a milling area, the sputtering rate increase depending on the incident angle of the ions, even if the ion dose is accurately measured, the milling depth deviates from the depth in the case where the flat area is milled. In addition, in the case as well where several layers are milled during the milling process, the deviation occurs.
In addition, in the case of an LSI or the like in which a wide wiring for power supply is distributed on the most upper layer, after having milled up the wiring for power supply, the wiring underlying the wiring for power supply needs to be milled. In this case, though the wiring layer is normally made of metal such as aluminium, this metal is polycrystalline. In this connection, since the sputtering rate is changed depending on the crystal orientation, when milling the wiring layer, the milling speed varies and hence it is difficult to decide the milling speed at this time. Therefore, while the accurate measurement of the ion dose is effective in the case of the flat and single milling object, with respect to the object having a complicated structure in which the wiring layer and the insulating layer are laminated into a plurality of layers as in an LSI, that accurate measurement becomes no more than the auxiliary means of the milling depth control. Thus, in order to mill the construction material to an accurate depth, it is necessary to recognize directly the milling state of the layer, which is being milled, at the milling time point.
The information of the layer which is being milled is contained in the secondary particles which are generated by the ion irradiation. The method of detecting light as the secondary particle is already put into practical use by some people, and is described in the above-mentioned article of Journal of Vacuum Science and Technology B6(6), November/December 1988, pp. 2100 to 2103.
Now, by the light is meant the light excited ion impact. This light excited ion impact is obtained in such a way that the atoms of the construction material of an LSI which are obtained by the sputtering are partially excited, and when such atoms are returned back to the ground state in the vicinity of the position where such atoms are emitted through the milled hole, such atoms emit the light having the wavelength inherent in the material of interest. However, in the case where both scale down (shrink) and the multi layer metallization are promoted as in LSIs of the recent years, the light emitted through the fine milled hole having a high aspect ratio is low in yield, and hence is difficult to be adopted for the practical judgement of the milling end point.
Furthermore, though the secondary electrons can be detected by an MCP (Micro Channel Plate) as a secondary particle detector, or a combination of a scintillator and a photomultiple tube which are generally mounted to the focused ion beam milling system, a photo detector needs to be additionally mounted thereto in order to detect the light. In actual, with respect to that mounting, when aiming at efficient collection of light, mounting that detector in a space defined by the distance between a lower end of a lens in the final stage of an ion optical system and a upper face of a stage as the beam focusing point which should come close to each other as much as possible in order to enhance the beam focusingness results in difficulty in the machine design as well as a sacrifice or limitation of the function of other assembly (e.g., the electron shower and the gas nozzle). Therefore, it is difficult to find out the merit with respect to the light detection and hence the light detection can not be adopted as the practical milling end point detecting means.
In the case where the surface is covered with an insulating layer made of silicon dioxide (SiO.sub.2) as in an LSI or the like, the positive electric charges are accumulated in the area which has been irradiated with the ion beam, and as a result, the so-called charge-up occurs. When the charged-up area is further irradiated with the ion beam, the positive electric charges of the ions and the positive electric charges on the charged-up surface repel each other so that the orbit of the ion beam is deflected and hence the position deviated from the milling position which is previously set will be milled. This problem is deadly to the milling and hence can not be admitted even when the application requiring the success yield is not of interest.
In addition, while there is the possibility that the voltage of the charge-up is increased up to the acceleration voltage of the ions, in actual, at a time point when the voltage of the charge-up has exceeded the breakdown voltage of the charged-up insulating film in the most upper layer, the breakdown is caused so that the electric charges are discharged towards the underlying wiring layer. In this connection, the traces of the breakdown ranging from the LSI surface to the wiring layer are formed and may become the paths through which the current leaks in some cases. When the excessive breakdown is caused, the electric charges charged-up on the insulating layer are spark-discharged to the wiring layer, and the insulating film overlying the wiring layer is scattered due to the impact of the breakdown. These phenomena are deadly when the LSI of interest is processed in the next process in order to be analyzed.
Then, as described above, the electron shower is installed, the electrons which have been drawn therefrom are applied to the vicinity of the milling area, and the positive electric charges of the ions are neutralized with the negative electric charges of the electrons, thereby avoiding the deflection of the beam due to the charge-up. At this time, since the electrons drawn from the electron shower need to be applied to an LSI, the electric field is directed to the direction of leading the electrons to the LSI side. In other words, when such an electric field is set, even if the secondary electrons which have been emitted by the ion beam irradiation go out from the LSI once, these secondary electrons are readily forced back to the LSI side. For this reason, when the neutralization of the electric charges is being carried out by the electron shower, it is impossible to detect any secondary electron. Therefore, while for an object of milling the surface of which is made of a conductor, it is possible to detect the secondary electrons to judge the milling end point, in the case where the coating of a conductor can not be applied to the surface of the normal LSI, the secondary electron can not be adopted as the means for judging the milling end point.
The secondary electrons have the yield which is the same order as that of the primary ion beam, whereas the yield of the secondary ions is in the range of 1 to 10% of that yield and hence is not high. However, the yield of the secondary electrons is higher than that of the light. In addition, even in the state in which the electron shower is applied to an LSI, the secondary electrons can be detected and hence the secondary electrons are the only practical detected particles in the detection of the milling end point in the LSI milling. In actual, the above-mentioned patent official gazettes (i.e., JP-B-3-28017, JP-A-58-202038, JP-B-5-14416 and JP-A-6-96712) describe that the secondary ions are made the means for detecting the milling end point.
However, in JP-B-3-28017, there is no concrete description with respect to the means for detecting the secondary ions. In JP-A-58-202038 and JP-B-5-14416, there is constructed the system provided with a mass analyzer as the secondary ion detector. In addition, while in JP-A-6-96712, there is described the method of carrying out both the element analysis and the structure analysis on the basis of the detection of the secondary ions, there is no practical description with respect to the detection of the milling end point in the milling of an LSI. That is, in the conventional milling end point detecting method of detecting the secondary ions, there is only disclosed the method of detecting the change during the milling in the ion detection amount of construction material forming the lamination layer, and hence there is not disclosed at all the method of solving the following three problems associated with the detection of the milling end point in the milling of an LSI by the actual focused ion beam.
(1) With respect to the structure of the wiring layer and the insulating layer of an LSI, not only a conductor such as aluminium is simply combined with a silicon dioxide, but also the various structures have been put into the practical use from the aspect of the device performance. In particular, in recent years, for the purpose of avoiding the electronmigration, there has been frequently adopted the method wherein the wiring layer made of aluminium is sandwiched between the layers each made of different material such as tungsten. However, there is not established yet the method of detecting the milling end point for a new wiring layer as having such a multilayer structure.
(2) In the milling of an LSI, the amount of materials to be milled is increased and hence the burden is increased which is imposed on an operator of a system for judging the milling end point while observing the secondary ions closely to stop the milling. In particular, while with respect to the process, such as a modification of the wiring, in which the milling extends over a long time period, the milling error is apt to occur, since in the application thereof, one mistake makes the chip of interest a defective, the milling yield is decreased and also the acquisition ratio of the non-defective chips to the constructed chips is reduced. In addition, in the application or the like wherein since the change in the signal which is used to judge the milling end point varies for every operator, a window is formed by boring through the wiring and metal is deposited to the surface of the chip while filling the window by the CVD method or the like in order to draw the wiring therefrom, the difference in the connection resistance occurs between the wiring of the chip itself and the wiring which is newly formed by the deposition of metal, and hence the dispersion occurs in the signal transfer speed of an LSI. This also results in the chip being made a defective and hence should be avoided.
(3) In the case of the milling process in which increasing the milling throughput is indispensable thereto, the beam having a large ion beam current is used in the milling. In this case, if the milling end point is not judged speedily, the milling progresses over a schedule and hence the milling yield is reduced. For example, when the aluminium wiring is milled with the beam of 2 nA, if the milling size is 1 .mu.m.quadrature., the progress of the milling depth for one second becomes approximately 1 .mu.m. This corresponds to the milling depth corresponding to the thickness of the wiring layer, and hence if the over-milling for one second is carried out, then even granted that a window is opened through the wiring, in actual, the window is opened throughout the wiring to mill even the underlying layer. In actual, while the excessive progress of the milling is mitigated to some degree by optimizing both the beam current to be used and the milling size, there is a limit to this technique due to the request from the throughput, and hence the milling end point should be judged within a short time period.
Hereinabove, the problems associated with the prior art have been described with the example of milling an LSI by the focused ion beam as the charged particles becoming the milling beam as the center of discussion. In this connection, the pattern formation by the fine milling of this sort is also applied to the electron beams. In addition, an object of the milling is not limited to an LSI, and hence it is also applied to an electronic device such as a liquid crystal display panel of the active matrix formula in which for example, thin film transistors (TFTs) are formed on a glass substrate with high density. Either case corresponds to the important pattern forming technology and also is the problem which is generally common to the electronic devices each having the multi-wiring layer structure in which the wiring layers and the insulating layers are laminated.